1. Field of the Invention
The present invention relates to speech coding, and more particularly, to a variable dimension spectral magnitude quantization apparatus and method using a predictive and mel-scale binary vector.
2. Description of the Related Art
The quantization of spectral magnitudes is a crucial issue in sinusoidal speech coding to obtain high quality low bit rate speech. There are two representative methods of quantizing a spectral magnitude. One is a method of quantizing a linear prediction (LP) spectral envelope using high order LP modeling and the other is a method of quantizing a LP spectral envelope and a residual spectral envelope using low order LP modeling and residual spectrum modeling. According to the former method, even though the order and the number of quantization bits increase, the improvement of performance converges into a consistent value, and the amount of computation or memory requirement is considerable. Accordingly, it is desired that a quantization method needing a small amount of computation or memory while improving the quality of speech is implemented with the application of the latter method.
The dimension of spectral magnitude is variable as it is sampled and estimated at the pitch harmonics. Several techniques have been suggested to quantize the spectral magnitude of variable dimension. A multiband excitation vocoder transforms a spectral magnitude into the coefficients of a discrete cosine transform, and then quantizes the coefficients using the combination of scalar and vector quantizers (DVSI, INMARSAT M Voice Codec, vol. 1.7. Digital Voice Systems Inc., September 1991). A sinusoidal transform coder represents a spectrum with the all-pole model of high order (R. J. McAulay and T. F. Quati, xe2x80x9cSinusoidal Codingxe2x80x9d, in Speech coding and synthesis (W. B. Kleijn and K. K. Paliwal, eds.), pp. 121-174, Amsterdam, The Netherlands: Elsevier, 1995). In band-limited interpolation (BLI), the variable-dimension of the spectrum is converted into a fixed-dimension based on sampling rate conversion and signal interpolation techniques (P. C. Meuse, xe2x80x9cA 2400 bps Multi-Band Excitation Vocoderxe2x80x9d, in Proc. Int. Conf. on Acoust., Speech, Signal Processing, pp. 9-12, 1990, and M, Nishiguchi, J. Matsumoto, R. Wakatsuki, and S. Ono, xe2x80x9cVector Quantized MBE with Simplified V/UV Decision at 3.0 kbpsxe2x80x9d, in Proc. Int. Conf. on Acoust., Speech, Signal Processing, pp. II 151-154, 1993). In variable-dimension vector quantization (VDVQ), a spectral vector is quantized directly using a universal codebook of a fixed-dimension (A. Das, V. Rao, and A. Gersho, xe2x80x9cVariable-Dimension Vector Quantizationxe2x80x9d, IEEE Signal Processing Letters, vol. 3 , pp. 200-202, July 1996). In non-squared transform vector quantization (NSTVQ), an input vector is transformed into a fixed dimension using a linear transform matrix (P. Lupini and V. Cuperman, xe2x80x9cVector Quantization of Harmonic Magnitude for Low-Rate Speech Codersxe2x80x9d, in Proc. Int. Conf. on Acoust., Speech, Signal Processing, pp. 858-862, 1994).
To obtain high spectral accuracy, however, these conventional techniques require not only a huge memory and a training step to keep and obtain the vector codebook, but also considerable search time to find an optimal code vector.
To solve the above problems, it is a first objective of the present invention to provide a variable dimension spectral magnitude quantization apparatus using a predictive and mel-scale binary vector, which quantizes a spectral magnitude with very low computational complexity and achieves high spectral accuracy by efficiently quantizing a residual spectral envelope using a predictive and mel-scale binary vector quantizer.
It is a second objective of the present invention to provide an apparatus for efficiently quantizing a residual spectral envelope in a variable dimension spectral magnitude quantization apparatus.
It is a third objective of the present invention to provide a method for efficiently quantizing a residual spectral envelope using predictive and mel-scale binary vector quantization in a procedure of a variable dimension spectral magnitude quantization.
It is a fourth objective of the present invention to provide a variable dimension spectral magnitude quantization method performed by the variable dimension spectral magnitude quantization apparatus.
Accordingly, to achieve the first objective, there is provided a variable dimension spectral magnitude quantization apparatus including a predictive quantizer for obtaining a predictive-quantized first residual spectral envelope from a quantized previous residual spectral envelope, a mel-scale binary vector quantizer for obtaining a second residual spectral envelope represented with a linear scale code vector using a mel-scale binary vector codebook, a synthesized spectral envelope generator for adding the output of the predictive quantizer and the output of the mel-scale binary vector quantizer to generate a quantized residual spectral envelope and multiplying the quantized residual spectral envelope by a corresponding quantized linear prediction spectral envelope to generate a synthesized spectral envelope, a comparator for comparing the synthesized spectral envelope with an original spectral envelope, and a minimum value detector for detecting a minimum value from the values sequentially obtained by the comparator.
To achieve the second objective, there is provided a residual spectral envelope quantization apparatus in a variable dimension spectral magnitude quantization apparatus. The residual spectral envelope quantization apparatus includes a predictive quantizer for obtaining a predictive-quantized first residual spectral envelope from a quantized previous residual spectral envelope, a mel-scale binary vector quantizer for obtaining a second residual spectral envelope represented with a linear scale code vector using a mel-scale binary vector codebook, and a residual spectral envelope quantizer for adding the output of the predictive quantizer and the output of the mel-scale binary vector quantizer to generate a quantized residual spectral envelope. The mel-scale binary vector codebook is used for representing a residual spectral envelope of a variable high dimension as a code vector of a fixed low dimension.
To achieve the third objective, there is provided a residual spectral envelope quantization method including the steps of (a) obtaining a predictive-quantized first residual spectral envelope from a quantized previous residual spectral envelope, (b) obtaining a second residual spectral envelope represented with a linear scale code vector using a mel-scale binary vector codebook, and (c) adding the first residual spectral envelope and the second residual spectral envelope to generate a quantized residual spectral envelope. The mel-scale binary vector codebook is used for representing a residual spectral envelope of a variable high dimension as a code vector of a fixed low dimension.
To achieve the fourth objective, there is provided a variable dimension spectral magnitude quantization method including the steps of (a) obtaining a predictive-quantized first residual spectral envelope from a quantized previous residual spectral envelope, (b) obtaining a second residual spectral envelope represented with a linear scale code vector using a mel-scale binary vector codebook, (c) adding the first residual spectral envelope and the second residual spectral envelope to generate a quantized residual spectral envelope and multiplying the quantized residual spectral envelope by a corresponding quantized linear prediction spectral envelope to generate a synthesized spectral envelope, (d) comparing the synthesized spectral envelope with an original spectral envelope, and (e) detecting a minimum value from the values sequentially obtained in the step (d).